Method for forming heat insulating film, and structure of heat insulating film

ABSTRACT

A method for forming a heat insulating film includes: a step of subjecting an aluminum alloy constituting a surface of a base material to an anodic oxidation treatment to form an anodic oxidation coating film having pores formed in the surface thereat a step of coating on the surface of the anodic oxidation coating film a sealing material that includes a silicon-based polymer solution and particles of a heat insulating material that are dispersed in the silicon-based polymer solution and are particles having an average particle diameter that is larger than an average pore diameter of the pores; and a step of drying and baking the sealing material to form a sealing coating film.

BACKGROUND

1. Field of the Invention

This invention relates to a method for forming a heat insulating film,and a structure of a heat insulating film.

2. Background Art

A method for forming a heat insulating film in an umbrella portion of anengine valve has already been disclosed in Japanese Patent Laid-Open No.2013-014830. Specifically, the aforementioned conventional methodincludes a first step of forming an aluminum plating film over theentire circumference of an engine valve, a second step of, afterformation of the aluminum plating film, subjecting the entirecircumference of the engine valve to an anodic oxidation treatment toform an anodic oxidation coating film, and a third step of, afterformation of the anodic oxidation coating film, subjecting an umbrellaportion of the engine valve to a sealing treatment to form a sealingcoating film. According to this conventional method, a heat insulatingfilm can be obtained that has a structure in which a sealing coatingfilm is formed on an anodic oxidation coating film. Further, accordingto the engine valve on which the above described heat insulating film isformed, in addition to improving the heat resistance and a heatinsulating property of a combustion chamber of the engine, a heatradiation property can also be improved.

Other prior arts include Japanese Patent Laid-Open No. 2012-047110,Japanese Patent Laid-Open No. 2013-060620, and Japanese Patent Laid-OpenNo. 2012-172619.

In this connection, when performing anodic oxidation treatment of analuminum alloy, there is the problem that because the formation of theanodic oxidation coating film is affected by inclusions that areincluded in the aluminum alloy, the surface of the anodic oxidationcoating film that is formed is not smooth, and minute concavities andconvexities arise thereon. This problem can also arise in a similarmanner in the aforementioned first and second steps in a case where analuminum alloy plating film is formed on the surface of the engine valveand the plating film is thereafter subjected to an anodic oxidationtreatment.

When concavities and convexities arise on the surface of an anodicoxidation coating film, a heat transfer area thereof increases. If theheat transfer area increases, an effect of improving the heat insulatingproperty that is obtained by the anodic oxidation coating film isweakened. If concavities and convexities have arisen on the surface ofthe anodic oxidation coating film, the fluidity of a flame that arisesinside the combustion chamber decreases, and the combustion efficiencydeteriorates. In this respect, by forming the sealing coating film inthe above described third step, the surface of the heat insulating filmhaving a structure in which the anodic oxidation coating film and thesealing coating film are formed can be made smooth to a certain extent.Ideally, it is desirable for the surface of the heat insulating film tobe made as smooth as the surface of the aluminum alloy prior to theanodic oxidation treatment.

In this connection, the sealing coating film is formed by subjecting asealing material that is the raw material of the sealing coating film toa drying and baking process. Consequently, in order to make the surfaceof the heat insulating film smooth by means of the sealing coating film,it is necessary to provide a large amount of the sealing material inconcave portions in the surface of the anodic oxidation coating film tothereby make the sealing material thick at such concave portions.However, because the sealing material contains a solvent, the thickerthat the sealing material is, the more difficult it becomes for a gas ofthe solvent that is generated at the time of drying and baking to escapeto the outside. Therefore, there is the problem that cracks are liableto arise in the sealing coating film. Consequently, there is a trade-offrelationship between thickening the sealing material to smoothen thesurface of the heat insulating film, and reducing cracks in the sealingcoating film, and it is difficult to achieve both a smooth surface and areduction in the amount of cracks in a compatible manner.

SUMMARY

The present invention has been conceived in view of the above describedproblem. That is, an object of the present invention is, with respect toa heat insulating film having a structure in which a sealing coatingfilm is formed on the surface of an anodic oxidation coating film, tosmooth the surface of the heat insulating film and also reduce theoccurrence of cracks in the sealing coating film in a compatible manner.

A first aspect of the present invention is a method for forming a heatinsulating film, including: a step of subjecting an aluminum alloyconstituting a surface of a base material to an anodic oxidationtreatment to form an anodic oxidation coating film having a surface inwhich pores are formed;

a step of coating on the surface of the anodic oxidation coating film asealing material that includes a silicon-based polymer solution andparticles of a heat insulating material that are dispersed in thesilicon-based polymer solution and are particles having an averageparticle diameter that is larger than an average pore diameter of thepores; and

a step of drying and baking the sealing material to form a sealingcoating film.

A second aspect of the present invention is in accordance with the firstinvention, wherein the particles may be particles that have a hollowstructure.

Further, in a third aspect of the present invention, an average primaryparticle diameter of the particles may be greater than 30 nm.

A fourth aspect of the present invention is a structure of a heatinsulating film that is formed by a formation method according to anyone of the first to third inventions, may including:

an aluminum alloy constituting a surface of a base material;

an anodic oxidation coating film that is formed on a surface of thealuminum alloy, and that has a surface in which pores are formed; and

a sealing coating film that is formed so as to cover an opening portionof the pores, and that includes particles of a heat insulating materialhaving an average particle diameter that is larger than an average porediameter of the pores.

A fifth aspect of the present invention is in accordance with the fourthinvention, wherein:

the particles may be particles that have a hollow structure; and

a porosity of the sealing coating film may be from 27.3 to 57.7%.

According to the first aspect of the present invention, a sealingtreatment can be performed using a sealing material that includes asilicon-based polymer solution and particles of a heat insulatingmaterial that are dispersed in the silicon-based polymer solution andare particles having an average particle diameter that is larger than anaverage pore diameter of pores of an anodic oxidation coating film. Inthe case of using a sealing material including particles of a heatinsulating material of the aforementioned size, the occurrence of cracksin a drying and baking process can be suppressed in comparison to whenusing a sealing material that does not include the particles. Therefore,the occurrence of cracks can be suppressed even when a sealing materialis made thicker by providing a large amount thereof on concave portionsof the surface of an anodic oxidation coating film. Further, the surfaceof the heat insulating film can be made smooth by means of a thicksealing coating film that is formed by drying and baking of the sealingmaterial.

According to the second aspect of the present invention, since a heatinsulating function of air inside particles that have a hollow structurecan be utilized, a heat insulating film can be formed that has a highheat insulating property in comparison to a heat insulating film thatdoes not include particles that have a hollow structure.

According to the third aspect of the present invention, a heatinsulating film that has a high heat insulating property can be formedby using particles which have an average primary particle diameter thatis greater than 30 nm.

According to the fourth aspect of the present invention, since a sealingcoating film is provided that is formed so as to cover an openingportion of pores of an anodic oxidation coating film, a structure of aheat insulating film having a high heat insulating property can beprovided that utilizes a heat insulating function of air inside thepores that is located at a deeper place than the opening portion.

According to the fifth aspect of the present invention, a structure of aheat insulating film having a high heat insulating property that isobtained by means of a sealing coating film in which the porosity isbetween 27.3 and 57.7% can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart for describing an embodiment of a method forforming a heat insulating film of the present invention;

FIG. 2 is a vertical cross-sectional view of an anodic oxidation coatingfilm;

FIG. 3 is a partially enlarged schematic view of the anodic oxidationcoating film shown in FIG. 2;

FIG. 4 is a vertical cross-sectional view of a heat insulating filmformed by the formation method of the embodiment;

FIG. 5A is a cross-sectional view of a heat insulating film formed usinga sealing material that does not include hollow silica particles;

FIG. 5B is a cross-sectional view of a heat insulating film formed usinga sealing material that does not include hollow silica particles;

FIG. 6A is a view illustrating a process for forming a sealing coatingfilm shown in FIG. 5;

FIG. 6B is a view illustrating a process for forming a sealing coatingfilm shown in FIG. 5;

FIG. 6C is a view illustrating a process for forming a sealing coatingfilm shown in FIG. 5;

FIG. 7 is a view for describing the periphery of a combustion chamber towhich a structure of a heat insulating film of the present invention isapplied;

FIG. 8 is a partially enlarged schematic view of the heat insulatingfilm shown in FIG. 7;

FIG. 9 is a view showing results of measuring a thermal conductivity λ;

FIG. 10 is a view showing results of measuring a volumetric heatcapacity C; and

FIG. 11 is a view showing results of measuring a surface roughness Ra.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereunder, an embodiment of a method for forming a heat insulating filmand of a structure of a heat insulating film according to the presentinvention will be described referring to FIG. 1 to FIG. 11. Note that,for each of the drawings, the same or corresponding portions are denotedby the same reference numerals, and a description of such portions issimplified or omitted.

[Method for Forming Heat Insulating Film]

First, an embodiment of the method for forming a heat insulating film ofthe present invention will be described. FIG. 1 is a flowchart fordescribing the embodiment of the method for forming a heat insulatingfilm. In the present embodiment, first, an anodic oxidation coating filmis formed on the surface of an aluminum alloy by subjecting a basematerial to an anodic oxidation treatment (step S1). In the presentstep, a treatment apparatus (not illustrated) that includes a flowchannel through which an electrolytic solution is circulated and a pairof electrodes is used. Further, in the present step, a base material isused that is made of aluminum alloy. However, instead of a base materialmade of aluminum alloy, a base material may be used in which an aluminumalloy plating film is formed on a surface such as heat-resisting steel,carbon steel, or a titanium material.

In step S1, specifically, the aforementioned base material is placed inthe aforementioned treatment apparatus, and a voltage is applied betweenthe aforementioned pair of electrodes while circulating an electrolyticsolution through the aforementioned flow channel. By this means, ananodic oxidation coating film is formed on the surface of the basematerial. The anodic oxidation coating film is a coating film of porousalumina that has a large number of pores that open at the surfacethereof (described in detail later). By means of this porous structure,the anodic oxidation coating film realizes a low thermal conductivityand a low heat capacity per unit volume (described in detail later).

However, the surface of the anodic oxidation coating film is rough incomparison to the surface of the base material before the anodicoxidation treatment. FIG. 2 is a vertical cross-sectional view of theanodic oxidation coating film. As shown in FIG. 2, concavities andconvexities have arisen on the surface of an anodic oxidation coatingfilm 10, and a surface roughness (arithmetic mean roughness) Ra thereofis an average of 4 to 5 μm. The reason concavities and convexities ariseon the surface of the anodic oxidation coating film 10 is thatinclusions that are contained in the aluminum alloy affect the formationof the anodic oxidation coating film 10. One object of steps S2 and S3that are described hereunder is to smooth the surface of a heatinsulating film in which the anodic oxidation coating film 10 is aconstituent element.

The description of the present embodiment will now be continuedreferring again to FIG. 1. After step S1, a sealing material is coatedonto the surface of the anodic oxidation coating film (step S2). In thepresent step, a sealing material is used that includes a silicon-basedpolymer solution that includes silicon in a main chain backbone (morespecifically, a polymer solution including polysilazane or polysiloxaneand an ether-based solvent), and silica particles that are dispersed inthe silicon-based polymer solution. The polymer solution may include anadditive as necessary. A dispersant that enhances the dispersibility ofthe particles, a leveling agent, a surfactant, a viscosity modifier andthe like may be mentioned as examples of the additive. The silicaparticles used in the present step have an average primary particlediameter (average particle diameter before the particles agglomerate andbecome secondary particles) that is larger than an average pore diameterof pores of the anodic oxidation coating film, and the silica particlesalso have a hollow structure. However, silica particles with a solidstructure may be used instead of the silica particles with a hollowstructure (hereunder, referred to as “hollow silica particles”), andparticles of a heat insulating material other than silica (for example,alumina (Al₂O₃), zirconia (ZrO₂), or titania (TiO₂) particles) may alsobe used. Further, two or more kinds among the aforementioned three kindsof particles may be used at the same time.

The average pore diameter of the pores of the anodic oxidation coatingfilm is approximately 30 nm. Therefore, in the present step, hollowsilica particles for which the average primary particle diameter isgreater than 30 nm (preferably, 50 nm) are used. However, a target valueof the surface roughness Ra of the heat insulating film that is formedby the present embodiment is approximately 1 Jam, and therefore in thepresent step hollow silica particles are used with respect to which anaverage secondary particle diameter is less than 1 μm (preferably 500nm, more preferably 150 nm).

Here, the term “average pore diameter” refers to an arithmetic meandiameter that is determined by photographing sectional images at aplurality of magnifications using a scanning electron microscope anddigitalizing the obtained images by a scanner input method, andthereafter calculating a distribution of diameters of circles having anarea that is equal to the area of respective pores extracted by computerimage analysis. Further, the term “average primary particle diameter”refers to an arithmetic mean diameter that is determined byphotographing transparent particle images at a plurality ofmagnifications using a transmission electron microscope and digitalizingthe obtained images by a scanner input method, and thereaftercalculating a distribution of diameters of circles having an area thatis equal to a projected area of respective pores extracted by computerimage analysis. Furthermore, the term “average secondary particlediameter” refers to an average particle diameter (D50 value) that isobtained by a dynamic scattering method, and is a diameter that can besimply measured by a commercially available particle size analysis andmeasurement apparatus.

The mixing ratio of the hollow silica particles in the sealing materialis appropriately adjusted in accordance with the target value (forexample, a value in a range from 27.3% to 57.7%) of the porosity of thesealing coating film to be formed after drying and baking of the sealingmaterial (after step S3).

The surface of the anodic oxidation coating film after application ofthe sealing material will now be described referring to FIG. 3. FIG. 3is a partially enlarged schematic view of the anodic oxidation coatingfilm 10 shown in FIG. 2. As shown in FIG. 3, the anodic oxidationcoating film 10 is constituted by alumina 10 a having a nonuniformlength in a perpendicular direction relative to the surface of thealuminum alloy, and pores 10 b. Further, a sealing material 12constituted by a silicon-based polymer solution 14 and hollow silicaparticles 16 is provided so as to cover an opening portion 10 c of thepores 10 b. A large amount of the sealing material 12 is provided atconcave portions of the surface of the anodic oxidation coating film 10,and a small amount of the sealing material 12 is provided at protrudingportions of the anodic oxidation coating film 10.

The description of the present embodiment will now be continuedreferring again to FIG. 1. A method for coating the sealing material instep S2 is not particularly limited, and examples thereof include aspray method, a blade coating method, a spin coating method, and a brushapplication method.

After step S2, the sealing material is dried and baked to form a sealingcoating film (step S3). The conditions (temperature, time period and thelike) at the time of drying and baking are appropriately adjusted inaccordance with the thickness of the sealing material that was coatedonto the surface of the anodic oxidation coating film. A heat insulatingfilm is formed by performing the present step. FIG. 4 is a verticalcross-sectional view of a heat insulating film that is formed by theformation method of the present embodiment. As shown in FIG. 4, asealing coating film 20 constituted by hollow silica particles 16 andsilica 18 derived from a silicon-based polymer is formed on the surfaceof the anodic oxidation coating film 10. A heat insulating film 22 isconstituted by the anodic oxidation coating film 10 and the sealingcoating film 20. A surface roughness Ra of the heat insulating film 22is equal to or less than 1 μm. The details of the structure of the heatinsulating film 22 as well as the effects obtained by the structure ofthe heat insulating film 22 will be described later.

The effects of the present embodiment will now be described referring toFIGS. 5A to 6C. Heat insulating films 30 a and 30 b shown in FIGS. 5Aand 5B are heat insulating films that were formed for the purpose ofcomparison with the heat insulating film 22. The heat insulating film 30a is constituted by a sealing coating film 32 a that does not includehollow silica particles, and the anodic oxidation coating film 10. Theheat insulating film 30 b shown in FIG. 5B is constituted by a sealingcoating film 32 b that does not include hollow silica particles, and theanodic oxidation coating film 10. The thickness of the sealing coatingfilm 32 b is greater than the thickness of the sealing coating film 32a, and is approximately equal to the thickness of the sealing coatingfilm 20 in FIG. 4. However, cracks 34 have arisen in the sealing coatingfilm 32 b.

FIG. 6A is a view illustrating a process for forming the sealing coatingfilm 32 a. FIG. 6B and FIG. 6C are views illustrating a process forforming the sealing coating film 32 b. In a case where a sealingmaterial 36 a that does not include hollow silica particles is thinlycoated (FIG. 6A), a drying rate of an upper part (surface part) of thesealing material 36 a at the time of drying and baking is approximatelyequal to a drying rate of an inner part of the sealing material 36 a.Consequently, gas of a solvent that is generated during drying andbaking is released from the inner part of the sealing material 36 a tothe outside thereof. In contrast, in a case where a sealing material 36b that does not include hollow silica particles is thickly coated (FIG.6B), the upper part of the sealing material 36 b hardens before theinner part of the sealing material hardens. Consequently, gas of asolvent that is generated during drying and baking cannot escape fromthe inner part of the sealing material 36 b, and cracks 34 arise in thesealing coating film 32 b (FIG. 6C).

As will be understood from FIGS. 5A to 6C, in the case of using asealing material that does not include hollow silica particles, there isthe problem that the thicker that the coating of the sealing materialis, the easier it is for cracks to arise in the sealing coating filmduring drying and baking (FIG. 5B, FIG. 6B and FIG. 6C). Further, whenthe sealing material is thinly coated, since a thin sealing coating filmis formed, the surface of the heat insulating film cannot be smoothedsufficiently (FIG. 5A and FIG. 6A). In contrast, according to thepresent embodiment, since the sealing material including hollow silicaparticles of the above described size is used, the gas of a solvent thatis generated during drying and baking can be released from inside thesealing material to the outside. The fact that it is easy for the gas ofthe solvent that is generated inside the sealing material to flow alongthe surface of the hollow silica particles to move to the upper part ofthe sealing material may be mentioned as one of the reasons why the gasof the solvent can be released. Accordingly, in a case where the sealingmaterial is thickly coated also, the generation of cracks can befavorably suppressed. Hence, a thick sealing coating film can be formedand the surface of the heat insulating film can be smoothed.

[Structure of Heat Insulating Film]

Next, an embodiment of the structure of a heat insulating film accordingto the present invention will be described. The structure of a heatinsulating film of the present invention is applied to an inner wall ofa combustion chamber of an engine. FIG. 7 is a view for describing theperiphery of a combustion chamber to which the structure of a heatinsulating film of the present invention is applied. Note that althoughFIG. 7 is described on the premise that the engine is a spark-ignitiontype engine, the structure of a heat insulating film of the presentinvention can also be applied to a compression-ignition type engine.

A cylinder 42 of an engine 40 is formed inside a cylinder block 44. Acylinder liner 46 is provided at an inner circumferential face of thecylinder 42. Further, inside the cylinder 42, a piston 48 is slidablydisposed with respect to the cylinder liner 46. A cylinder head 50 isinstalled at an upper portion of the cylinder block 44. An intake port52 and an exhaust port 54 are formed in the cylinder head 50. An intakevalve 56 is provided in the intake port 52, and an exhaust valve 58 isprovided in the exhaust port 54.

A space that is surrounded by an inner circumferential face of thecylinder liner 46, a top face of the piston 48, a bottom face of thecylinder head 50, a bottom face of an umbrella portion of the intakevalve 56 and a bottom face of an umbrella portion of the exhaust valve58 corresponds to a combustion chamber 60. That is, an inner wall of thecombustion chamber 60 is constituted by the inner circumferential faceof the cylinder liner 46, the top face of the piston 48, the bottom faceof the cylinder head 50, the bottom face of the umbrella portion of theintake valve 56 and the bottom face of the umbrella portion of theexhaust valve 58. The heat insulating film 22 formed by the abovedescribed method is provided on the inner wall of the combustion chamber60.

FIG. 8 is a partially enlarged schematic view of the heat insulatingfilm 22 shown in FIG. 7. As shown in FIG. 8, the heat insulating film 22has a structure that includes the anodic oxidation coating film 10 andthe sealing coating film 20. The anodic oxidation coating film 10 isconstituted by the alumina 10 a and the pores 10 b. The sealing coatingfilm 20 is constituted by the hollow silica particles 16 and the silica18, and is formed so as to cover the opening portions 10 c.

The silica 18 has a lower thermal conductivity than the aluminum alloy,and has a lower heat capacity per unit volume (volumetric heat capacity)than the aluminum alloy. Further, the alumina 10 a has a lower thermalconductivity and a lower volumetric heat capacity than not only thealuminum alloy, but also than a conventional ceramic-based heatinsulation material. Therefore, by applying the structure of the heatinsulating film 22, in addition to improving the heat resistance and theheat insulating property of the combustion chamber 60, a heat radiationproperty thereof can also be improved.

Further, according to the structure of the heat insulating film 22including the hollow silica particles 16 of the above described size,the heat insulating property of the combustion chamber 60 can be furtherimproved. The reason for this will now be described in detail referringto FIG. 9 to FIG. 11. FIG. 9 is a view showing results of measuring athermal conductivity λ of two kinds of heat insulating films. FIG. 10 isa view showing results of measuring a volumetric heat capacity C of thetwo kinds of heat insulating films. The thermal conductivity λ and thevolumetric heat capacity C were calculated based on the followingequations after measuring a specific heat capacity Cp and a thermaldiffusivity α with respect to two kinds of samples (a sample containinghollow silica particles and a sample that did not contain hollow silicaparticles).

X=Cp×ρ×α

C=Cp×ρ

Where, Cp represents specific heat capacity, ρ represents density, and αrepresents thermal diffusivity.

The sample containing hollow silica particles (hereunder, referred to as“sample A”) was prepared as follows. First, a base material (test pieceof aluminum alloy) was subjected to an anodic oxidation treatment toform an anodic oxidation coating film. Next, hollow silica particles(hollow silica particles manufactured by GRANDEX Co., Ltd (primaryparticle diameter 90 to 110 nm)) were mixed in a polysilazane solution(ingredients and percentages: diethyl ether 72%, poly(perhydrosilazane)20%, and anisole 8%) and stirred adequately using a stirrer to therebyprepare a sealing material. Thereafter, the sealing material was appliedfive times onto the anodic oxidation coating film using a brush, andthen dried and baked for 8 hours in a constant temperature oven at 180°C. to thereby prepare the sample A. The sample that did not contain thehollow silica particles (hereunder, referred to as “sample B”) wasprepared in the same manner as the sample A except that the polysilazanesolution was used as the sealing material.

The measurement conditions and the like for the specific heat capacityCp and the thermal diffusivity c were as follows.

(1) Specific Heat Capacity Cp

Measurement method: DSC method

Measurement apparatus: DSC Q1000 manufactured by TA Instruments

Reference sample: Sapphire

Measurement atmosphere: N₂ atmosphere

Measurement sample: After processing each sample to Φ6 mm, the basematerial was dissolved in hydrochloric acid to prepare samples that wereconstituted only by film

(2) Thermal Diffusivity α

Measurement method: Laser flash method

Measurement apparatus: LFA 457 manufactured by NETZSCH

Temperature measurement method: Noncontact temperature measurement usingInSb sensor

Surface treatment: Graphite spray

Measurement atmosphere: N₂ atmosphere

Calculation technique: Base material and film were measured in anintegrated state, and the thermal diffusivity of only the film wascalculated by multilayer analysis including pulse width correction andheat loss.

The measurement results in FIG. 9 and FIG. 10 are shown as percentagesbased on the sample B as 100%. As shown in FIG. 9 and FIG. 10, thethermal conductivity λ of the sample A (with particles) is low incomparison to the sample B (without particles), and the volumetric heatcapacity C of the sample A is also lower than the sample B. Theseresults indicate that the heat insulating property of the sample A issuperior to that of the sample B. The fact that the sample A includeshollow silica particles and the air in the internal space of the hollowsilica particles functions similarly to the air inside the pores 10 bmay be mentioned as one of the reasons why the heat insulating propertyof the sample A is superior to that of the sample B.

The fact that the surface roughness Ra of the sample A is small may bementioned as another reason why the sample A has an excellent heatinsulating property. FIG. 11 is a view illustrating results of measuringthe surface roughness Ra. The surface roughness Ra was measured withrespect to both the sample A and the sample B that were prepared in thesame manner as described above. However, with respect to the sample A,three different kinds of samples were prepared using three kinds ofsealing materials that were prepared by changing the mixing ratio of thehollow silica particles. The porosity (=volume of internal space ofhollow silica particles/volume of sample×100) after drying and bakingthe three kinds of sealing materials was as follows.

Sample A1: 27.3% (porosity: low)

Sample A2: 46.3% (porosity: medium)

Sample A3: 57.7% (porosity: high)

The surface roughness Ra was measured in accordance with JIS B601(2001). The measurement results in FIG. 11 are shown as percentagesbased on the sample B as 100%. As shown in FIG. 11, the surfaceroughness Ra of the samples A1 to A3 (with particles) was less than thatof the sample B (without particles). Further, the surface roughness Raof the sample A3 was less than that of the sample AI and the sample A2.The fact that the surface roughness Ra is small means that the surfaceof the relevant sample is smooth and a heat transfer area is small.Accordingly, it was found that the heat insulating property of thesample A is superior to that of the sample B. Further, it was found thatthe heat insulating property of the sample A3 is superior to that of thesample A1 and the sample A2.

1. A method for forming a heat insulating film, comprising: a step ofsubjecting an aluminum alloy constituting a surface of a base materialto an anodic oxidation treatment to form an anodic oxidation coatingfilm having a surface in which pores are formed; a step of coating onthe surface of the anodic oxidation coating film a sealing material thatincludes a silicon-based polymer solution and particles of a heatinsulating material that are dispersed in the silicon-based polymersolution and are particles having a primary particle diameter that islarger than an outer diameter of the pores; and a step of drying andbaking the sealing material to form a sealing coating film.
 2. Themethod for forming a heat insulating film according to claim 1, whereinthe particles are particles that have a hollow structure.
 3. The methodfor forming a heat insulating film according to claim 1, wherein theprimary particle diameter of the particles is greater than 30 nm.
 4. Astructure of a heat insulating film that is formed by a formation methodaccording to claim 1, comprising: an aluminum alloy constituting asurface of a base material; an anodic oxidation coating film that isformed on a surface of the aluminum alloy, and that has a surface inwhich pores are formed; and a sealing coating film that is formed so asto cover the surface of the anodic oxidation coating film, and thatincludes particles of a heat insulating material having a primaryparticle diameter that is larger than an outer diameter of the pores. 5.The structure of a heat insulating film according to claim 4, wherein:the particles are particles that have a hollow structure; and a porosityof the sealing coating film is from 27.3 to 57.7%.